Process of making plastic calcined gypsum



. manufacture of neat gypsum the 1919 report quoted above, it is recom- UNITED STATES PATENT OFFICE.

E. EMLEY, OF WASHINGTON, DISTRICT OI COLUMBIA.

PROCESS OF MAKING PLASTIC CALCINED GYPSUM.

Io Drawing.

Patented Oct. 4, 1921.

Serial No. 442,818.

(FILED UNDER THE ACT OF MARCH 3, 1833, 22 STAT. It, 625.)

To all whom it may concern:

Be it known that I, WARREN E. EMLEY, a citizen of the United States, residin at Washington, in the District of Colum ia, have invented new and useful Improvements in the Process of Making Plastic Calcined Gypsum, of which the following is a specification.-

- Thisapplication is filed under the act of March 3, 1883, under the terms of which, the applicant agrees that the invention described herein, if patented, may be used by the Government or any of its oflicers or employees in rosecution of work for the Government, or

y any other person in the United States without the payment to him of any royalty thereon.

Calcined gypsum, or plaster of Paris, is a chemical compound of the formula CaSO,.%H,O. It is of crystalline structure and sandy character. Because of this characteristic, calcined gypsum cannot be used alone for the white or finishin coat of plaster: the plasterer is not physlcally able to spread it. For the same reason, not more than two or three parts (by weight) of sand may be added to it in the preparation of the scratch or brown coats of plaster. This characteristic is described in the trade as non-plastic.

It is quite possible to make a material sufiiciently plastic for white coat work from calcined gypsum by the addition of other materials. It is known, for instance, that the addition of molasses, of certain other organic materials, or of loamy sand, will improve the plasticity of the calcined ypsum. In the 1919 Tentative Report on 1)- sum- Plasters, issued by the American Society for Testing Materials, it is stated that a neat gypsum plaster may contain up to 15 per cent. of hydrated lime, ground clay, asbestos, retarder, or fiber. In the 1920 report, this was changed to permit the addition of 15 per cent. of materials to control the working quality, setting time and fibering. It is general trade practice to add hydrated lime to calcined gypsum 1n the plaster. In

mended that laster for white coat work be composed of 5 per cent. lime putty and 25' per cent. calcined gypsum. Practically all white coat plaster is composed of these two ingredients, although the proportions of them are variable.

If calcined gypsum is ground after cal cination, one of two things may occur: If the grinding is not severe, the crystals will be reduced in size, but the sandy or nonplastic nature of the material willstill remain. If the grinding is very severe, a chemical change can be made to occur, the water being actually ground out of the material. In actual practice, a buhr mill or tube mill is used for this grinding, and any Water which is liberated is evaporated and carried off by the current of air passing through the mill. The resultant product is known as soluble anhydrite. This is anhydrous calcium sulfate, but it differs from the naturally occurring anhydrite in that it has a great aflinity for water. So great is this aflinity that a few moments exposure to moist air is sufiicient for it to recombine with enough water to change back to the original calcined gypsum. Because of this fact, and also because the soluble anhydrite is itself crystalline, the efiect of this severe grinding is lost, and the product is still nonplastic. Commercial gypsum plasters consist mostly of calcined gypsum, with more or less soluble anhydrite and/or undecomposed gypsum.

I find that if calcined gypsum is ground severely so as to liberate the water, but in such a way that the water cannot escape, the resultant product has radically different properties. It is now plastic, rather than non-plastic. This can be roved by testing the material by means of t e Carson blotter test as described in the Tramsactions of the National Lime Manufacturers Association, 1916, p. 175, or by means of a plasticimeter. It can be used alone as the white coat of plaster, and can be used with at least six arts (by weight) of sand for the scratch or rown coats. When compared with the original material, it will be found to require more water to mix it to a given consistency, that it sets more slowly, and that it will develop higher strength after setting. To use th1s material as a wall plaster, it will,

of course, be necessary to retard it, using cined gypsum in such a way as to prevent evaporation of the, liberated water, the same end can be attained by adding, either before or during the grinding, enough water to make up for the quantity evaporated, so that the final product will contain approximately one molecule of water to two molecules of calcium sulfate.

At a meeting of Committee 0-7 on Lime, of the American Society for Testing .Materials, held on January 12, 1921, directions were adopted for the use of the plasticimeter described in Bureau of Standards Technologic Paper N o. 16.9, and for the calculation of plasticity figures. It was also decided that a lime putty, to be satisfactory for finishing purposes (white-coat work) must have a plasticlty figure of not less than 200.

This plastic calcined gypsum may be produced from substances other than calcined sum. The same end product will result i raw gypsum is ound in such a way that some, but not al of its water is allowed to escape; or if the requisite proportion of water is ground into anhydrite, either soluble or natural.

Specifically, I find that if calcined gypsum is ground in a ball mill or similar sealed container, with such severity as to liberate the water of crystallization, the resultant product will be at least as plastic as finishing hydrated lime. This grinding may be either in addition to or in lieu of the usual grinding which is a part of the regular process of manufacture of calcined sum. The duration of the grinding, WhlCh determines the plasticity of the finished product, depends upon the quality (fineness) of the original material, and also upon the size of the mill. In a small laboratory mill, it required 7 hours to produce one pound of material of the desired plasticity; in a larger mill, it required 4 hours for 80 pounds of material. I find that plastic calcined gypsum can be made by this method from calcined gy sum from any source, and that storage or four months does not cause any noticeable deterioration of its plasticity.

I claim:

1. The process of increasing the plasticity of calcined gypsum which comprises the grinding of the calcined gypsum and preventig gl escape of its water content.

2. e process of increasing the plasticity water content during the grinding of calcined gypsum which comprises grinding the calcined gypsum and maintaining its water content.

3. The rocess of increasing the plasticit ofcalcine d gypsum which consists in grin ing the calcined psum to eliminate the operation, preventing the escape of the said water content during the grinding step, and con tinuously keeping the above treated gypsum in contact with the water thus eliminated and permitting the water thus eliminated to be reabsorbed before ceasing the grinding operation. I

4. The process of increasing the plasticit of calcined psum which comprises grind ing the calcined psum, with the addition, before the grin ing operation, of enough water to make up for that lost by evaporation during the grinding.

5. The process of increasing the plasticit of calcined gypsum which comprises grin ing the calcined gypsum, with the addition, he ore the cessation of the grinding operation, of enough water to make up for that lost IX evaporation during the grinding.

6. plastic calcined psum which, when properly retarded, w give a plasticity figure of at least 200.

7. A plastic calcined gypsum having a plasticit figure of at least 200, the plasticity 0 which is not dependent upon the addition of any foreign material.

8. A plastic calcined gypsum having a plasticity figure of at least 200, the. plasticity of which is not dependent upon the addition of lime.

9. A plastic calcined gypsum, having a plasticity figure of at least 200, the plasticity of which is not de endent upon the addition of other forms 0 calcium sulfate.

10. A plastic gy sum plaster, containing calcium sulfate and water in approximately the same proportions as found in the hemlhydrate, uncombined, with'a high degree of plasticity.

11. A plastic gypsum plaster containing calcium sulfate and water in approximatel the same proportions as found in the hemihydrate, uncombined, with a high degree of P asticity, independent of the addition of.

WARREN E. EMLEY. 

